1. Technical Field
The present invention relates to a development roller having a roughness on the circumference thereof for transporting toner to a latent image bearing unit, a development device containing the development roller, and an image forming apparatus containing the development device.
2. Related Art
Development devices developing a toner image from a latent image with one-component non-magnetic toner triboelectrically charge the toner on a development roller. A development roller known in the related art (such as the one disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2007-121948) has a surface roughness on the circumference thereof, the roughness having a substantially flat top surface. With the surface roughness, the development roller triboelectrically charges the toner thereon. As illustrated in FIG. 7A, a development roller a includes a base unit b and a surface layer c plated on the base unit a as a coverage.
The development roller a generally remains in contact with a toner feed roller and a toner regulator (both not shown). Silica having a high hardness is used serving as an external additive that coats toner mother particles of the toner.
A roughness, composed a plurality of recesses m and projections e, is formed on the circumference of the base unit b. Edges are formed at the g of a top flat area f of the projection e. A roughness, composed of plurality of recesses h and projections i, is formed on the circumference of the surface layer c. Edges are formed at sides k of a top flat area j of the projection i.
The surface layer c is worn by the toner feed roller and the toner regulator in an image forming operation. The edges formed at the sides k of the projection i of the surface layer c are worn at a localized manner. As the image forming cycles increase, the projection i of the surface layer c of the development roller a is worn away and rounded in a curved surface as illustrated in FIG. 7B. The edges are formed at the sides k of the flat area j of the projection j. If the projection j of the surface layer c is worn and rounded, the sides g of the flat area f of the projection e become rapidly exposed. If part of the base unit is exposed, the charging property of the toner on the development roller a is lowered, and it becomes difficult to perform efficiently a charging operation. If a low-cost iron (Fe) based material is used for the base unit b, the exposure of the base unit can lead to corrosion. If the base unit b is exposed early, the service life of the development roller a is shortened. There is room for improvement in the durability of the development roller a. Even if the sides k of the projection portion j are not edged, a portion at the sides k may be worn in a localized fashion. The same problem may still be expected.